acoustics - How to calculate the sound pressure at a given distance of a point source?

Consider a point source in a free field. The source is emitting spherical waves. I know the relationship ${p_1\over r_2}={p_2\over r_1}$ that lets you convert the sound pressure $p_1$ at a point at a distance $r_1$ to the point source the sound pressure $p_2$ at a point at a distance $r_2$ from the point source.

But how can you calculate the sound pressure at a given distance from a point source if you are given the sound pressure at the point source? The corresponding distance would be zero...